1. Field of the Invention
The present invention relates generally to chambers used in semiconductor processing, and more specifically to a method and apparatus for removing contaminants from chambers to enhance performance of the chambers.
2. Background of Related Art
Thin film deposition apparatus and techniques are used, among other techniques, to provide thin film layers on semiconductor substrates. One well known prior art deposition process is physical vapor deposition, generally referred to as sputtering, wherein substrates are typically moved through load locks and into high vacuum processing chambers which enclose the substrate and a target composed of the material desired to be deposited on the substrate. A negative voltage applied to the target excites into a plasma state an inert gas (typically argon) supplied to the chamber, and ions from the plasma bombard the target and eject particles of target material which deposit on the substrate to form the desired film. In some sputtering process applications, the substrate may be heated to temperatures on the order of about 350xc2x0 C. to about 510xc2x0 C. or higher, to reflow a film layer deposited on the substrate by sputtering.
The low pressure, high temperature environments typically utilized in deposition processes cause out gassing of contaminants such as hydrogen (H2), water (H2O) and air (mostly O2 and N2), from the substrates and from the internal walls of the processing chamber. These contaminants, are released slowly and are often detrimental to the film layer which is deposited onto the substrate.
Contaminants which are out gassed from the processing chamber walls should also be removed from the processing chambers to improve the processing environment. However, out gassing of the chamber walls continues for many hours and it is not feasible to reduce the chamber pressure sufficiently to draw out all of the contaminants prior to processing of wafers in the processing chamber. A typical method for reducing out gassing from chamber walls of a new chamber is to bake out the contaminants by initially reducing the chamber pressure substantially below an acceptable processing pressure and raising the chamber temperature above an acceptable processing temperature for about 28 hours. Following bake out, the chamber pressure is raised to the processing pressure by adding a clean gas, and the chamber temperature is lowered to the processing temperature. The increase in chamber pressure and decrease in chamber temperature substantially slows down the out gassing of contaminants.
U.S. Pat. No. 5,536,330 describes sweeping of contaminants from a vacuum chamber during bake out by flowing preheated argon through the chamber at a pressure above 50 Torr. The argon is preheated to a temperature of at least 90xc2x0 C., preferably from about 150 to 250xc2x0 C. The argon sweep is performed for a sufficient amount of time to remove contaminants prior to reducing the pressure within the chamber and testing for remaining contaminants. The bake out of a 5 liter chamber can be completed in 15 hours when combined with a hot argon sweep if the chamber is repeatedly swept with argon and pumped to a low pressure. After bake out, the chamber is cooled for over 18 hours prior to processing of semiconductor substrates.
U.S. Pat. No. 5,678,759 describes gas compressors configured to heat a purge gas for sweeping contaminants from a chamber. The gas compressors directly heat the gas molecules which minimizes heating of chamber components and thus reduces cooling time.
Several days are typically required to remove contaminants from a 5 liter chamber with a hot argon sweep and then cool down the chamber to processing temperatures at a chamber pressure less than 10xe2x88x928 Torr. The time required can be reduced by heating the gas with a series of gas compressors, and thus reducing the time required for chamber cooling. However, considerable time is still required to evacuate the chamber after each argon sweep to purge the contaminants. Therefore, there is a need for a method for removing contaminants from the processing chamber walls in less time to improve productivity of the processing chambers.
The present invention provides a method for preparing a vacuum chamber prior to semiconductor processing. The method includes the step of flowing a non-reactive gas, preferably argon, through the vacuum chamber at a pressure below about 4 Torr to sweep contaminants from the vacuum chamber. The chamber pressure is reduced to less than about 100 mTorr after sweeping with argon, and the sweeping cycle is repeated as needed to remove contaminants. The gas, the chamber or both are heated during subsequent sweep cycles to enhance removal of contaminants. Following the sweep cycles, remaining contaminants are baked out at a chamber pressure less than about 6xc3x9710xe2x88x926 Torr. The method substantially removes moisture and other contaminants from the chamber in less than 8 hours, and prepares the chamber for processing in less than one day.